Linkage of the Circumglobal Teleconnection and the Interannual Variability of Early Spring Diabatic Heating over Low-Latitude Highlands in Southeast Asia

Yu Yang aYunnan Key Laboratory of Meteorological Disasters and Climate Resources in the Greater Mekong Subregion, Yunnan University, Kunming, China

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Dayong Wen aYunnan Key Laboratory of Meteorological Disasters and Climate Resources in the Greater Mekong Subregion, Yunnan University, Kunming, China

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Jie Cao aYunnan Key Laboratory of Meteorological Disasters and Climate Resources in the Greater Mekong Subregion, Yunnan University, Kunming, China

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https://orcid.org/0000-0003-0256-761X
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Abstract

This study explores the linkage of the circumglobal teleconnection (CGT) on the variability of early spring diabatic heating over the Southeast Asian low-latitude highlands (SEALLH) using ERA5 data. The early spring diabatic heating over the SEALLH shows significant interannual variability with a quasi-3-yr period. Anomalies in the advection of the early spring diabatic heating in the troposphere over the SEALLH associated with CGT are mainly responsible for the interannual variability of early spring diabatic heating over the SEALLH. When CGT is in phase with an anomalous cyclone over the eastern midlatitude North Atlantic, an anomalous cyclone usually dominates the west SEALLH throughout the troposphere. Stronger-than-normal southerly winds located on the east flank of the anomalous cyclone in the lower–upper troposphere transport more high-enthalpy air mass from lower latitudes to the SEALLH and then result in stronger-than-normal early spring diabatic heating over the SEALLH. When CGT is in phase with an anomalous anticyclone over the eastern North Atlantic, the opposite conditions occur, and weaker-than-normal early spring diabatic heating is observed over the SEALLH. Such significant correlation between CGT and early spring diabatic heating over the SEALLH can persist from winter to early summer. The key physical processes revealed in the observational analysis are mostly confirmed by the historical simulation performed with the EC-EARTH3 model.

Significance Statement

The low-latitude highlands in Southeast Asia are one of the earliest diabatic heating sources in the Asian summer monsoon region. Variability of diabatic heating over the low-latitude highlands in Southeast Asia significantly regulates the weather and climate over the Asian summer monsoon region. However, the interannual variability of early spring diabatic heating over the low-latitude highlands in Southeast Asia remains unclear. This study determines that the circumglobal teleconnection links with the interannual variability of early spring diabatic heating over the low-latitude highlands in Southeast Asia via modulating the local advection process from the previous winter. These results build a bridge connecting the anomalous signals occurring in the upper reaches of the low-latitude highlands in Southeast Asia with the weather and climate in the local and lower reaches of the low-latitude highlands in Southeast Asia.

Yu Yang, Dayong Wen, and Jie Cao are equal first coauthors.

© 2023 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Jie Cao, caoj@ynu.edu.cn

Abstract

This study explores the linkage of the circumglobal teleconnection (CGT) on the variability of early spring diabatic heating over the Southeast Asian low-latitude highlands (SEALLH) using ERA5 data. The early spring diabatic heating over the SEALLH shows significant interannual variability with a quasi-3-yr period. Anomalies in the advection of the early spring diabatic heating in the troposphere over the SEALLH associated with CGT are mainly responsible for the interannual variability of early spring diabatic heating over the SEALLH. When CGT is in phase with an anomalous cyclone over the eastern midlatitude North Atlantic, an anomalous cyclone usually dominates the west SEALLH throughout the troposphere. Stronger-than-normal southerly winds located on the east flank of the anomalous cyclone in the lower–upper troposphere transport more high-enthalpy air mass from lower latitudes to the SEALLH and then result in stronger-than-normal early spring diabatic heating over the SEALLH. When CGT is in phase with an anomalous anticyclone over the eastern North Atlantic, the opposite conditions occur, and weaker-than-normal early spring diabatic heating is observed over the SEALLH. Such significant correlation between CGT and early spring diabatic heating over the SEALLH can persist from winter to early summer. The key physical processes revealed in the observational analysis are mostly confirmed by the historical simulation performed with the EC-EARTH3 model.

Significance Statement

The low-latitude highlands in Southeast Asia are one of the earliest diabatic heating sources in the Asian summer monsoon region. Variability of diabatic heating over the low-latitude highlands in Southeast Asia significantly regulates the weather and climate over the Asian summer monsoon region. However, the interannual variability of early spring diabatic heating over the low-latitude highlands in Southeast Asia remains unclear. This study determines that the circumglobal teleconnection links with the interannual variability of early spring diabatic heating over the low-latitude highlands in Southeast Asia via modulating the local advection process from the previous winter. These results build a bridge connecting the anomalous signals occurring in the upper reaches of the low-latitude highlands in Southeast Asia with the weather and climate in the local and lower reaches of the low-latitude highlands in Southeast Asia.

Yu Yang, Dayong Wen, and Jie Cao are equal first coauthors.

© 2023 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Jie Cao, caoj@ynu.edu.cn

1. Introduction

The low-latitude highlands (mean altitude > 1000 m) are located between 30°S and 30°N. According to this definition, there are 10 main low-latitude highlands worldwide (Xie and Liu 1998; Qin et al. 1997). The Southeast Asian low-latitude highlands (SEALLH) comprise the Hengduan Mountains, Yun–Gui Plateau, and their southern extension in northeastern Myanmar, northern Laos, and northern Vietnam. The area of the SEALLH is ∼900 000 km2 (Fig. 1), equivalent to one-third of the Tibetan Plateau (TP). Since the column-integrated diabatic heating over the SEALLH generally transitions from a heat sink to a heat source in March, one month earlier than over the Tibetan Plateau (Fig. 2), the SEALLH are one of the earliest diabatic heating sources in the Asian summer monsoon region (Chen and Li 1985).

Fig. 1.
Fig. 1.

(a) Location and (b) topography of the SEALLH.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0659.1

Fig. 2.
Fig. 2.

Annual cycle of column-integrated diabatic heating over the SEALLH (W m−2). Values for TP are shown for comparison (blue bars). Column-integrated diabatic heating over the SEALLH averaged in the region (18°–30°N, 96°–108°E) with altitude exceeding 1000 m (purple solid line in Fig. 1a). Column-integrated diabatic heating over the TP averaged in the region 25°–45°N, 65°–105°E with altitude exceeding 3000 m.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0659.1

Although the SEALLH have a relatively smaller area and lower altitude than the TP, the SEALLH can significantly modulate weather and climate over the Asian summer monsoon region. For example, some studies have indicated that water vapor streams from the Bay of Bengal (BOB) and the South China Sea (SCS) merge in the SEALLH, carry water vapor into the region east of the SEALLH, and ultimately regulate droughts and floods over East Asia (Xu et al. 2004; Cao et al. 2012, 2017; Day et al. 2015). Some studies illustrated the effect of the northern part of the SEALLH, the Yun–Gui Plateau, on the Asian summer monsoon. Zhang et al. (2015) demonstrated that the uplift of the northern SEALLH somewhat intensifies the circulation of the East Asian summer monsoon by arising an eastward extension of cyclonic circulation around the TP. He et al. (2016) suggested that surface radiative heating over the main body of the SEALLH is of primary importance in the formation of the nocturnal low-level jet along the eastern foothills of the Yun–Gui Plateau. Shi et al. (2017) suggested that the main part of the SEALLH, the Yun–Gui Plateau, reduces summer precipitation over the Indian monsoon region by ∼38%. Some studies also found that the precipitation variation over Southeast Asia is directly modulated by the anomalous meridional wind resulting from the interaction between the monsoonal circulation and local topography over the southern SEALLH (Chang et al. 2005; Xie et al. 2006; Qi and Wang 2012; Wang and Chang 2012; Wu et al. 2014, 2018; Wu and Hsu 2016; Zhuang et al. 2022). However, in contrast to abundant studies on the variability of diabatic heating over the Tibetan Plateau and its climatic effects (e.g., Flohn 1957; Yeh et al. 1957; Yanai et al. 1992; Li and Yanai 1996; Wang et al. 2008; Duan and Wu 2008; Wu et al. 2012; Rajagopalan and Molnar 2013; Zhao et al. 2018; He et al. 2019; Xie and Wang 2021, and references cited therein), there has been relatively little research investigating early spring diabatic heating over the SEALLH. The spatiotemporal evolution of early spring diabatic heating over the SEALLH remains unclear.

The circumglobal teleconnection (CGT), a zonal wavenumber-5 pattern, is one of the most prominent teleconnections in the Northern Hemisphere during boreal winter (Branstator 2002; Watanabe 2004), and a similar pattern is found in boreal summer (Ding and Wang 2005). The CGT has significant impacts on the climate variability over the entirety of the Northern Hemisphere (Sung et al. 2006; Ding and Wang 2007; Feldstein and Dayan 2008; Huang et al. 2011; Chen and Huang 2012; Saeed et al. 2014; Wang et al. 2013; Almazroui et al. 2018; Yu et al. 2019; Chen et al. 2019). For example, Saeed et al. (2011a,b) suggested that the positive phase of CGT usually favors a stronger Indian summer monsoon by strengthening monsoon trough–like anomalies over northwestern India and Pakistan. Wu et al. (2016) found that CGT can also modulate the interdecadal variation of the East Asian summer monsoon. Xue and Chen (2019) found that the northern mode of the South Asian high can be influenced by a stronger CGT via the atmospheric response to positive rainfall anomalies over the Indian Peninsula. Recent studies suggested that CGT-related circulation anomalies can account for precipitation anomalies over the southeastern Tibetan Plateau and the southern slope of Tibetan Plateau near the SEALLH (Huang et al. 2018; Wang et al. 2022). Particularly, Zhang et al. (2021) found that extreme heat events over the southeastern edge of the Tibetan Plateau are closely linked to a spring-type circumglobal teleconnection pattern.

These motivate us to investigate the impact of CGT on the interannual variability of early spring diabatic heating over the SEALLH. We attempt to answer the following questions: Is CGT related to the variability of early spring diabatic heating over the SEALLH? If so, what are the corresponding physical processes? We start with a description of the data and methods used in this study.

2. Data and methods

We use ERA5 data provided by the European Centre for Medium-Range Weather Forecasts (Hersbach et al. 2020) for the period 1979–2019. ERA5 has 1° × 1° resolution in latitude and longitude. We further analyzed the historical CMIP6 simulation (r1i1p1f1) performed with the EC-EARTH3 model covering the period 1970–2014. The EC-EARTH3 model has a T255 horizontal resolution (grid size ∼ 80 km) and 91 vertical levels (Döscher et al. 2022).

The diabatic heating is calculated using the equation
Q1=Cp(pp0)κ(θt+Vθ+ωθp),
where θ denotes the potential temperature, ω and V are the vertical and horizontal velocities, p is the pressure, p0 is a reference pressure of 1000 hPa, Cp is the specific heat capacity of dry air at a constant pressure, κ=R/Cp, and R is the gas constant of dry air. The operator 〈⋯〉 denotes vertical integration [ (1/g)ptps()dp], where ps is Earth’s surface pressure, pt is 125 hPa, and g is the gravitational acceleration (Yanai et al. 1992; Li and Yanai 1996). In Eq. (1), 〈θt〉 reflects the contributions of the column-integrated local tendency, 〈Vθ〉 is the contribution from the column-integrated advection, and 〈ωθp〉 is the contribution of the column-integrated vertical transfer to the column-integrated diabatic heating.
The wave-activity flux is used to diagnose stationary wave propagation (Takaya and Nakamura 1997). The wave-activity flux W is expressed as
W=p*2|U|{U(υ2ψυx)+V(uυ+ψux)U(uυ+ψux)+V(u2+ψuy)f0RdN2H0[U(υTψTx)+V(uTψTy)]},
where U = (U, V) denotes the horizontal background flow velocity, p* is pressure normalized by 1000 hPa, ψ′ represents the perturbed geostrophic streamfunction, u′ and υ′ indicate the perturbed geostrophic winds, Rd is the gas constant of dry air, H0 is the constant scale height, f0 = 9.87 × 10−5 s−1 represents the Coriolis parameter at 43°N, N2 denotes the Brunt–Väisälä frequency, and T′ is the perturbed temperature.
The moist enthalpy, comprising temperature and humidity, is calculated as
h=CpT+Lυq,
where T is temperature, Lυ is the latent heat of water vaporization, and q is specific humidity (Wu et al. 2017). In this study, the column-integrated moist enthalpy fluxes and its divergence are defined as
Fh=Vh,
Dh=Fh.
Empirical orthogonal function (EOF) analysis (Preisendorfer 1988) and power spectrum analysis were adopted to reveal the interannual variability of early spring diabatic heating over the SEALLH. We also used regression analysis and Student’s t test to distinguish the crucial physical processes. Early spring in this study refers to March–April (MA).

3. Results

a. Spatiotemporal evolution of early spring diabatic heating over the SEALLH

After performing an EOF analysis on the detrended early spring diabatic heating over the SEALLH (18°–30°N, 96°–108°E), we found that the explained variances of the first four EOF modes are 55.7%, 14.3%, 10.7%, and 5.5%, respectively. Because the explained variance of the first EOF mode (EOF1) is distinctly larger than those of other modes, EOF1 reflects the most prominent characteristics of the detrended early spring diabatic heating over the SEALLH. Here, we only focus on the spatiotemporal characteristics of EOF1. The EOF1 of early spring diabatic heating features a monopole pattern with a maximum exceeding 0.9 over the SEALLH (Fig. 3a). The normalized principal component of EOF1 (PC1) changes year by year (Fig. 3b). The peaks of the power spectral density of the normalized time series PC1 are mainly concentrated in the period below 9 years, and only the power spectral density around the quasi-3-yr period passes the significance test at the 90% confidence level (Fig. 3c). Apparently, the evolution of the detrended early spring diabatic heating over the SEALLH is concentrated on an interannual time scale.

Fig. 3.
Fig. 3.

(a) Spatial pattern of EOF1 of the early spring diabatic heating over the SEALLH. (b) PC1 of EOF1. (c) Power spectrum of PC1. In (a), the purple solid line denotes the boundary of SEALLH. Shading from light to dark indicates the 90%, 95%, and 99% confidence levels based on the two-tailed Student’s t test. In (c), the thick solid line indicates the 90% confidence level with 6 degrees of freedom.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0659.1

b. Relationship between CGT and early spring diabatic heating over the SEALLH and possible physical processes

To reveal the relationship between CGT and detrended early spring diabatic heating over the SEALLH, we first perform an EOF analysis on the early spring 300-hPa nondivergent meridional wind anomalies following the method used by Branstator (2002). The EOF1 of the early spring 300-hPa nondivergent meridional wind is easily obtained. Its explained variance is 23.6%. Figure 4a shows that there are four anomalous centers in EOF1: two negative centers with values exceeding −0.6 around the Mediterranean–northern Africa and western Indian subcontinent, and two positive centers with values exceeding 0.6 around the Arabian Peninsula and SEALLH–south Indochina Peninsula. After defining the time series associated with the EOF1 of early spring 300-hPa nondivergent meridional wind as CGTI-1, the early spring 300 hPa horizontal winds can be regressed onto the CGTI-1. Figure 4b shows an obvious circumglobal circulation pattern with zonal 5 waves. Five anomalous anticyclones dominate western Europe, the southeastern Arabian Peninsula, Philippines, Bering Strait, and northeastern North America, separated by five anomalous cyclones over eastern North Africa, the northeastern Indian subcontinent, northwest Pacific, northwest coast of North America, and Greenland. The correlation coefficient between CGTI-1 and PC1 of the detrended early spring diabatic heating over the SEALLH reaches 0.86. Because the critical value is only 0.55 at the 99.9% confidence level with 30 effective degrees of freedom (Bretherton et al. 1999), the correlation coefficient is significant above the 99.9% confidence level.

Fig. 4.
Fig. 4.

(a) Spatial pattern of EOF1 at 300 hPa. (b) Regression of the 300-hPa horizontal winds (vectors; m s−1) onto the normalized CGTI-1. (c) Normalized CGTI-1 associated with EOF1 (blue line) and normalized principal component associated with diabatic heating (black line) over the SEALLH in early spring. In (a) and (b), the purple solid line denotes the boundary of the SEALLH. Red (blue) from light to dark indicates that positive (negative) correlation coefficient is significant at 90%, 95%, and 99% confidence levels with 39 degrees of freedom.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0659.1

To distinguish the possible physical processes underlying the interannual variability of the early spring diabatic heating over the SEALLH, the horizontal winds at 700, 500, and 200 hPa in early spring were regressed onto the normalized CGTI-1 (Fig. 5) in the same period. The most pronounced feature at 700 hPa is a wave train extending from midlatitude North Atlantic to East Asia in early spring (Fig. 5a). Three anomalous cyclones, separated by two anomalous anticyclones over western Europe and the southeastern Arabian Peninsula, dominate the eastern North Atlantic, eastern Mediterranean, and northeastern Indian subcontinent. The same alternating pattern with three anomalous cyclones and two anomalous anticyclones can be seen in the midtroposphere (Fig. 5b) and upper troposphere (Fig. 5c) from midlatitude North Atlantic to East Asia in early spring. The intensities of the three anomalous cyclones and two anomalous anticyclones, characterized by the anomalous horizontal winds, gradually increase from the lower to upper troposphere, but their positions remain almost unchanged. Such a vertical distribution indicates that the wave train has a quasi-barotropic structure. The 200-hPa wave-activity flux regressed onto the normalized CGTI-1 shows significant eastward propagation in midlatitude North Atlantic that extends southeastward to the Mediterranean, and then is split into two branches over the Anatolia Plateau. One branch of the wave activity propagates to mid–high latitudes, and the other branch continues to propagate southeastward to the Arabian Peninsula, eastward to the Indian subcontinent, then northeastward around the eastern Bay of Bengal before finally reaching northeast Asia in early spring (Fig. 5d).

Fig. 5.
Fig. 5.

Regression of the (a) 700-, (b) 500-, (c) 200-hPa horizontal winds (vectors; m s−1), and (d) 200-hPa wave-activity flux (vectors; m−2 s−2) onto the normalized CGTI-1. Shading from light to dark indicates the 90%, 95%, and 99% confidence levels based on the two-tailed Student’s t test. The purple solid line indicates the boundary of the SEALLH. In (a), anticyclones are labeled as A and cyclones are labeled as C.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0659.1

The anomalous circulation patterns regressed onto the normalized PC1 of the detrended early spring diabatic heating over the SEALLH (Fig. 6) are in phase with those regressed onto CGTI-1 (Fig. 5). In fact, all spatial correlation coefficients exceed 0.89 (Table 1). The spatial correlation coefficient with the lowest magnitude (0.89) appears in the correlation between the 200-hPa horizontal wind regressed onto the normalized CGTI-1 and that regressed onto the normalized PC1 of the detrended early spring diabatic heating over the SEALLH. After considering the effective degrees of freedom, the lowest spatial correlation coefficient still passes the significance test above the 99.9% confidence level.

Fig. 6.
Fig. 6.

As in Fig. 5, but regressed onto the normalized PC1 of the early spring diabatic heating over the SEALLH.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0659.1

Table 1

Spatial correlation coefficients between horizontal winds associated with early spring diabatic heating and those associated with EOF1 of CGT in the lower, middle, and upper troposphere.

Table 1

Figure 7a shows the long-term mean distribution of column-integrated moist enthalpy. In early spring, the highest moist enthalpy air masses appear over the lower-latitude and/or lower-altitude areas, with a maximum exceeding 2.2 × 109 J m−2. The moist enthalpy decreases with the increment of latitude and/or altitude. For example, the moist enthalpy is only ∼1 × 109 J m−2 over the northwestern SEALLH. The moist enthalpy fluxes at 700, 500, and 200 hPa in early spring were regressed onto the normalized CGTI-1. The results show similar patterns to those associated with horizontal winds. An anomalous cyclone with the quasi-barotropic structure covers the northeastern Indian subcontinent, and the SEALLH are dominated by southerly anomalies from the lower troposphere to the upper troposphere (Figs. 7b–d). The cyclonic anomalies and southerly anomalies also can be observed in anomalous column-integrated moist enthalpy fluxes associated with CGTI-1 (Fig. 7e). The anomalous southerly winds in the entire troposphere result in higher-enthalpy air mass from the eastern BOB–western SCS converged over the SEALLH (Fig. 7f).

Fig. 7.
Fig. 7.

(a) The long-term mean distribution of column-integrated moist enthalpy (shading; 109 J m−2). Regression of the (b) 700-, (c) 500-, (d) 200-hPa horizontal moist enthalpy fluxes (vectors; 105 J m kg−1 s−1), (e) column-integrated moist enthalpy fluxes (vectors; 109 J m−1 s−1), and (f) divergence of horizontal moist enthalpy fluxes (shading; 102 J m−2 s−1) onto the normalized CGTI-1. Shading from light to dark indicates the 90%, 95%, and 99% confidence levels based on the two-tailed Student’s t test for (b)–(e). Stippling indicates the 90% confidence levels based on the two-tailed Student’s t test in (f). The purple solid line indicates the boundary of the SEALLH.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0659.1

These results indicate that when CGT is in phase with an anomalous cyclone in the eastern North Atlantic, an anomalous cyclone usually dominates the west SEALLH, and an anomalous anticyclone usually controls the east SEALLH in the lower–upper troposphere. Stronger-than-normal southerly winds, located between the anomalous cyclone and anticyclone in the lower–upper troposphere, result in stronger-than-average early spring diabatic heating over the SEALLH via transporting more high-enthalpy air mass into the same region. On the contrary, when CGT is in phase with an anomalous anticyclone in the eastern North Atlantic, an anomalous anticyclone generally controls the west SEALLH, and an anomalous cyclone mainly occupies the east SEALLH throughout the troposphere. Northeasterly anomalies between the anomalous anticyclone and cyclone mostly reduce higher-enthalpy air mass into the SEALLH and then lead to weaker-than-normal early spring diabatic heating over the SEALLH.

To illustrate the relative contributions of column-integrated advection, column-integrated vertical transfer, and column-integrated local tendency associated with CGT to the interannual variability of early spring diabatic heating over the SEALLH, the explained variances of CGTI-1 were calculated for early spring diabatic heating and its three components, respectively (Fig. 8). Figure 8a shows the contribution of the explained variances of CGTI-1 to the interannual variability of early spring diabatic heating over the SEALLH. Most of the SEALLH except for its northwestern part is covered by the explained variances exceeding 60%. The explained variances of CGTI-1 for the interannual variability of column-integrated advection over the SEALLH (Fig. 8b) are slightly lower than those related to early spring diabatic heating over the SEALLH. Most of the SEALLH are covered by the explained variances exceeding 50%, and the middle–eastern SEALLH are dominated by the explained variance exceeding 60%. The explained variances of CGTI-1 for the interannual variability of column-integrated vertical transfer over the SEALLH (Fig. 8c) are lower than those related to early spring diabatic heating and column-integrated advection. The centers with the explained variances exceeding 20% appear mainly in the southwestern SEALLH. The explained variances of CGTI-1 for the interannual variability of column-integrated local tendency over the SEALLH are the lowest (Fig. 8d). The explained variances of CGTI-1 are less than 5% over the whole SEALLH during early spring. These results indicate that anomalous advection and vertical transport associated with early spring CGT significantly modulate the interannual variability of early spring diabatic heating over the SEALLH.

Fig. 8.
Fig. 8.

Explained variance ratios of the normalized CGTI-1 contributing to (a) early spring diabatic heating, (b) column-integrated advection, (c) column-integrated vertical transfer, and (d) column-integrated local tendency. The purple solid line indicates the boundary of the SEALLH.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0659.1

c. Persistence of the relationship of CGT, early spring diabatic heating, and its components over the SEALLH

To reveal the persistence of the relationship of CGT with early spring diabatic heating and its three components over the SEALLH, we first project daily 300-hPa nondivergent meridional wind anomalies from 1 January to 31 December each year onto the EOF1. The daily CGTI-1 is easily obtained in 1979–2019. Subsequently, the lead–lag correlation coefficients of early spring diabatic heating and its components averaged over the SEALLH can be calculated using the 61-day running-mean CGTI-1 (Fig. 9).

Fig. 9.
Fig. 9.

Lead–lag correlation coefficients of CGTI-1, early spring diabatic heating, and its three components averaged over the SEALLH. Cyan, red, blue, green, and brown curves denote correlation coefficients associated with CGTI-1 itself, early spring diabatic heating, column-integrated advection, column-integrated vertical transfer, and column-integrated local tendency, respectively. Dashed lines denote the critical values significant at the 95% confidence level.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0659.1

Figure 9 shows that the lead–lag correlation coefficients between CGTI-1 in early spring and the 61-day running-mean CGTI-1 (cyan line) gradually increase in the previous winter, reaching a maximum in the early spring, and then tend to decrease until early the following winter. The correlation coefficients in February–June are significantly above the 95% confidence level (dashed lines). This result indicates that the CGT patterns associated with the CGTI-1 can persist for nearly six months from winter to early summer and tend to disappear as the winter pattern of atmospheric circulation turns into the summer pattern.

The lead–lag correlation coefficients (red line) between early spring diabatic heating averaged over the SEALLH and the 61-day running-mean CGTI-1 share similar performance, but the period in which the correlation coefficients are significantly above the 95% confidence level becomes a little shorter and ends around May. The lead–lag correlation coefficients (blue line) between column-integrated advection averaged over the SEALLH and the 61-day running-mean CGTI-1 come nearest to the lead–lag correlation coefficients between early spring diabatic heating in the early spring and the 61-day running-mean CGTI-1 (red line), and the correlation coefficients significant above the 95% confidence level persist from January to May. The significant correlation between column-integrated vertical transfer averaged over the SEALLH and the 61-day running-mean CGTI-1 (green line) persists from January to April, slightly shorter than those associated with early spring diabatic heating and column-integrated advection. However, the correlation coefficients (brown line) associated with column-integrated local tendency do not pass the significance test at the 95% confidence level. These results, agreeing well with the previous results, indicate that the significant relationship between CGT and interannual variability of early spring diabatic heating can persist from winter to spring.

d. Model results

To keep the phase of the CGT spatial modes consistent with the observation, the simulated CGTI-1 (CGTI-1S) was calculated by projecting the March–April 300-hPa nondivergent meridional wind anomalies of EC-EARTH3 onto the observational EOF1 (Fig. 10a). The correlation coefficients between CGTI-1S and simulated early spring diabatic heating are positive over the SEALLH (Fig. 10a). The regions passing the significance test at the 90% confidence level are mainly located in the western, northeastern, and southeastern SEALLH. Compared with the correlation coefficients between CGTI-1 and early spring diabatic heating over the SEALLH (Fig. 10b), the simulation is able to reproduce the positive correlation pattern with differences in magnitude (Fig. 10). The simulated results confirm the observational linkage between CGT and early spring diabatic heating over the SEALLH.

Fig. 10.
Fig. 10.

Correlation coefficients (a) between simulated early spring diabatic heating anomalies and CGTI-1S and (b) between observational early spring diabatic heating anomaly and CGTI-1. Stippling indicates the 90% confidence levels based on the two-tailed Student’s t test. The purple solid line indicates the SEALLH.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0659.1

To verify the associated physical processes revealed in the observations, horizontal winds at 700, 500, and 200 hPa simulated by EC-EARTH3 are also regressed onto CGTI-1S (Fig. 11). Figure 11 shares similar patterns to the observations to a larger degree (Fig. 5). In early spring, a wave train, extending from midlatitude North Atlantic to East Asia, can be observed at 700 hPa (Fig. 11a). Herein, three anomalous cyclones, divided by two anomalous anticyclones over southwestern Europe and the southeastern Arabian Peninsula, control the northeastern North Atlantic, eastern Mediterranean, and northeastern Indian subcontinent. The same quasi-barotropic pattern, alternating three anomalous cyclones with two anomalous anticyclones from midlatitude North Atlantic to East Asia, can be also found in the middle and upper troposphere (Figs. 11b,c). Note that southerly anomalies, prevailing between the eastern flank of the anomalous cyclone over the northeastern Indian subcontinent and western flank of the anomalous anticyclone over the east SEALLH, are caught in the simulation from lower to upper troposphere. Of course, there are some differences between observation and simulation. For example, the northeastern North Atlantic is covered by southeasterly anomalies in the observation but prevails southwesterly anomalies in the simulation. Easterly anomalies in simulation around northern Africa are much stronger than in observation. These results, mainly catching the southeasterly anomalies over the SEALLH, confirm the robustness of the key physical process, i.e., when CGT is in the phase with southerly anomalies over the SEALLH, the southerly anomalies result in stronger early spring diabatic heating by inputting a higher-enthalpy air mass into the SEALLH.

Fig. 11.
Fig. 11.

Regression of the EC-EARTH3 model simulated (a) 700-, (b) 500-, and (c) 200-hPa horizontal winds (vectors; m s−1) onto the normalized CGTI-1S. Shading from light to dark indicates the 90%, 95%, and 99% confidence levels based on the two-tailed Student’s t test. The purple solid line indicates the boundary of the SEALLH. In (a) anticyclones are A and cyclones are C.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0659.1

4. Summary

Using ERA5 data for the period 1979–2019, we have explored the interannual variability of early spring diabatic heating over the SEALLH and the key physical processes underlying its interannual variability. The leading mode of early spring diabatic heating over the SEALLH is obtained using EOF analysis. The EOF1, characterized by a monopole structure over the entirety of the SEALLH, plays a dominant role in the spatiotemporal evolution of early spring diabatic heating over the SEALLH. A power spectral analysis of the first principal component linked with the EOF1 shows that most of the variability of early spring diabatic heating over the SEALLH occurs on an interannual time scale.

The associated physical processes, determined by regressing early spring atmospheric circulation, early spring diabatic heating, and its components onto the time series of CGTI-1 and PC1, indicate that the different phases of CGT in early spring mostly modulate the interannual variability of early spring diabatic heating over the SEALLH. The analysis results suggest that anomalies in early spring diabatic heating advection caused by CGT, especially those in the meridional direction in the lower–upper troposphere, make the most important contribution to the interannual variability of early spring diabatic heating over the SEALLH. When CGT is in phase with an anomalous cyclone over the eastern North Atlantic, an anomalous cyclone usually dominates the west SEALLH in the lower–upper troposphere. Stronger-than-normal southerly winds, located between the eastern flank of the anomalous cyclone and western flank of the anomalous anticyclone in the lower–upper troposphere, transport more high-enthalpy air mass from the eastern BOB–western SCS to the SEALLH and result in stronger-than-average early spring diabatic heating over the SEALLH. On the contrary, when CGT is in phase with an anomalous anticyclone over the eastern North Atlantic, an anomalous anticyclone generally controls the west SEALLH in the lower–higher troposphere. Northeasterly anomalies over the east flank of the anomalous anticyclone mostly block high-enthalpy air from converging into the SEALLH. Consequently, weaker-than-average early spring diabatic heating can be observed over the SEALLH. Of course, anomalies in early spring diabatic heating vertical transport and sensible heating flux also make a relatively important contribution to the interannual variability of early spring diabatic heating over the SEALLH.

The historical simulation of the EC-EARTH3 model confirmed the key physical process linking CGT and early spring diabatic heating over the SEALLH in the interannual time scale. The model results successfully reproduced the quasi-barotropic structure of the anomalous CGT wave train and associated southerly anomalies dominating the Indochina Peninsula–SEALLH. Such a pattern favors higher-enthalpy air mass being transported northward and eventually leads to an enhanced early spring diabatic heating over the SEALLH.

The significant lead–lag self-correlations of CGTI-1 agree with previous studies in which the winter pattern of CGT is established with the westerly jet split into two branches upstream of the TP; the southern branch stretches across South Asia and disappears when the westerly jet jumps northward around June (Branstator 2002; Watanabe 2004). The persistence of the significant correlations, in which CGTI-1 leads early spring diabatic heating and its three components averaged over the SEALLH, suggests that CGTI-1 could be adopted as a predictor of early spring diabatic heating over the SEALLH. However, the persistence of the significant correlations, in which CGTI-1 lags early spring diabatic heating and its three components averaged over the SEALLH, implies that early spring diabatic heating may also exert an impact on CGT in early summer, and it may then become a possible node triggering the onset of the Asian summer monsoon. Previous studies also found that CGT can be observed in boreal summer (Ding and Wang 2005) and significantly modulates the East Asian summer monsoon (Zhou et al. 2020).

Some studies indicated that the teleconnections over the mid–high latitudes of Eurasia can influence the precipitation and temperature in East Asia (Xu et al. 2019; Chen et al. 2020). The 200-hPa wave-activity flux regressed onto the normalized CGTI-1 shown in Fig. 5d, agreeing with their results, suggests the covariability of north and south branches of the wave-activity flux may influence weather and climate over the SEALLH and its lower reaches.

Other studies indicated that ENSO or Indian or Atlantic Ocean sea surface temperature anomalies are responsible for the interannual variability of rainfall over the Indochina Peninsula (Ge et al. 2021; Wen et al. 2021; Gui et al. 2021). However, Wen et al. (2021) suggest that the EOF1 of summer precipitation over the Indochina Peninsula shows a meridional dipole pattern, and the significant anomalies of precipitation associated with sea surface temperature anomalies mainly cover central–southern Indochina Peninsula. In addition, the interannual variability of diabatic heating is not entirely determined by latent heat associated with precipitation. Therefore, the correlation coefficients of the indices of SST, PC1 of early spring diabatic heating over the SEALLH, and CGTI-1 fail to pass the significance test at the 90% confidence level in neither the same period nor the previous winter (Table 2). These differences imply that early spring diabatic heating over the SEALLH may not be totally contributed to by the individual latent, sensible, or radiation heating. It would be valuable for future studies to investigate whether early spring diabatic heating anomalies over the SEALLH are linked with the Asian summer monsoon and CGT in boreal summer. It would be valuable to further investigate the three issues.

Table 2

The correlation coefficients of the Niño-3.4 index (Trenberth and Stepaniak 2001), Indian Ocean basin mode (IOBM) index (Saji et al. 2006), Atlantic meridional mode (AMM) index (Chiang and Vimont 2004), PC1 of the early spring diabatic heating over the SEALLH, and CGTI-1.

Table 2

Acknowledgments.

This work was supported by the National Natural Science Foundation of China (42030603) and Joint Foundation Project between Yunnan Science and Technology Department and Yunnan University (Grants 2019FY003017).

Data availability statement.

The ERA5 data were downloaded from https://climate.copernicus.eu/climate-reanalysis. The historical CMIP6 simulation (r1i1p1f1) performed with the EC-EARTH3 model is downloaded from https://esgf-node.llnl.gov/search/cmip6.

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  • Almazroui, M., M. Alobaidi, S. Saeed, A. Mashat, and M. Assiri, 2018: The possible impact of the circumglobal wave train on the wet season dust storm activity over the northern Arabian Peninsula. Climate Dyn., 50, 22572268, https://doi.org/10.1007/s00382-017-3747-1.

    • Search Google Scholar
    • Export Citation
  • Branstator, G., 2002: Circumglobal teleconnections, the jet stream waveguide, and the North Atlantic Oscillation. J. Climate, 15, 18931910, https://doi.org/10.1175/1520-0442(2002)015<1893:CTTJSW>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Bretherton, C. S., M. Widmann, V. P. Dymnikov, J. M. Wallace, and I. Bladé, 1999: The effective number of spatial degrees of freedom of a time-varying field. J. Climate, 12, 19902009, https://doi.org/10.1175/1520-0442(1999)012<1990:TENOSD>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Cao, J., J. Hu, and Y. Tao, 2012: An index for the interface between the Indian summer monsoon and the East Asian summer monsoon. J. Geophys. Res., 117, D18108, https://doi.org/10.1029/2012JD017841.

    • Search Google Scholar
    • Export Citation
  • Cao, J., W. Zhang, and Y. Tao, 2017: Thermal configuration of the Bay of Bengal–Tibetan Plateau region and the May precipitation anomaly in Yunnan. J. Climate, 30, 93039319, https://doi.org/10.1175/JCLI-D-16-0802.1.

    • Search Google Scholar
    • Export Citation
  • Chang, C.-P., Z. Wang, J. McBride, and C.-H. Liu, 2005: Annual cycle of Southeast Asia—Maritime Continent rainfall and the asymmetric monsoon transition. J. Climate, 18, 287301, https://doi.org/10.1175/JCLI-3257.1.

    • Search Google Scholar
    • Export Citation
  • Chen, G., and R. Huang, 2012: Excitation mechanisms of the teleconnection patterns affecting the July precipitation in northwest China. J. Climate, 25, 78347851, https://doi.org/10.1175/JCLI-D-11-00684.1.

    • Search Google Scholar
    • Export Citation
  • Chen, L., and W. Li, 1985: The atmospheric heat budget in summer over Asia monsoon area. Adv. Atmos. Sci., 2, 487497, https://doi.org/10.1007/BF02678747.

    • Search Google Scholar
    • Export Citation
  • Chen, S., R. Wu, L. Song, and W. Chen, 2019: Interannual variability of surface air temperature over mid-high latitudes of Eurasia during boreal autumn. Climate Dyn., 53, 18051821, https://doi.org/10.1007/s00382-019-04738-9.

    • Search Google Scholar
    • Export Citation
  • Chen, S., R. Wu, W. Chen, K. Hu, and B. Yu, 2020: Structure and dynamics of a springtime atmospheric wave train over the North Atlantic and Eurasia. Climate Dyn., 54, 51115126, https://doi.org/10.1007/s00382-020-05274-7.

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